Monday, October 29, 2012

1210.7086 (S. R. K. Rodriguez et al.)

Bosons (particles with integer spin) above a critical density to temperature ratio may macroscopically populate the ground state of a system, in an effect known as Bose-Einstein Condensation (BEC). The observation of BEC in dilute atomic gases was a great triumph of modern physics, a task requiring nK cooling of atoms. Following these demonstrations, a quest for lighter bosons enabling BEC at higher temperatures came to light. Photons in a microcavity were destined to fulfil this quest. Their coupling to semiconductor excitons allowed the condensation of exciton-polaritons at a few K in solid-state, and the condensation of photons was later observed in a liquid-state dye at room-temperature. Distinctly, one of the most actively studied excitations in condensed matter, surface plasmon polaritons - collective oscillations of conduction electrons in metals -, has never been shown or predicted to exhibit BEC. The strong radiative and Ohmic losses in metals, together with the lack of a suitable (e.g. harmonic) potential for thermalisation, are likely the reasons for this. Here we demonstrate BEC in a plasmonic system for the first time. Surface plasmon polaritons in a periodic array of metallic nanorods couple strongly to excitons in a room-temperature solid-state dye acting as a heat bath, and bosonic quasiparticles known as plexcitons are formed. By increasing the plexciton density through optical pumping, we observe the thermalisation and ground state accumulation of plexcitons in the angular spectrum and in real-space. Jointly, polarization build-up of the emission takes place. A new state of light-matter emerges upon plexciton condensation, and a coherent radiation field emanates from this quantum phase transition. We find the plexciton condensate to be the warmest and least massive of any condensate yet reported.

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